TITLE: GCN CIRCULAR NUMBER: 17336 SUBJECT: GRB 150120B: GROND Afterglow Detection DATE: 15/01/21 09:14:34 GMT FROM: Philip Wiseman at MPE/Swift P Wiseman, J Graham, P Schady and J Greiner (All MPE Garching) report on behalf of the GROND team: We observed the field of GRB 150120B (Swift trigger 627170; D'Elia et al., GCN 17314) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile). Observations started at 01:25 UT on 21.01.2015, ~18hrs after the GRB trigger. They were performed at an average seeing of 1.1" and at an average airmass of 1.7. We find a single point source within the 1.4" Swift-XRT error circle (Goad et al., GCN 17320), consistent with that reported by Perley et al. (GCN 17315), Guidorzi et al. (GCN 17316), at RA (J2000.0) = 02 h 37 m 09.77 s DEC (J2000.0) = +08d 04' 40.49" with an uncertainty of 0.3" in each coordinate. Based on 25 min of total exposures in g'r'i'z'JHK, we estimate preliminary magnitudes and 3 sigma upper limits (all in AB system) of g' > 24.2 r' = 23.6 +/- 0.1 mag, i' = 22.7 +/- 0.2 mag, z' = 22.4 +/- 0.40 mag, J = 20.2 +/- 0.3 mag, H = 19.9 +/- 0.5 mag, and K = 18.7 +/- 0.4 mag. Based on these magnitudes, the best-fit SED has a break in the g' band suggesting a photometric redshift of 3.5 +/-0.5. Given magnitudes are calibrated against GROND zeropoints as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.17 mag in the direction of the burst (Schlegel et al. 1998).